How do you tell your Cartridge needs replacing?

You should listen for signs of mis-tracking on the most demanding points of tracks with very sharp and loud transients, such as where a female singer gets too close to the microphone and really hits the note hard.  If you start to hear a fuzzy sound or strange noise where this never happened before, you have mis-tracking (the stylus is no longer in proper contact with the groove and is slamming the grooves instead of tracing the grooves.  When the stylus is worn or the suspension has gone bad, mis-tracking increases.  Before this point, it is hard to hear signs of wear because they happen so gradually.

I don’t think that microscopes are that useful for anything other than seeing gross wear, or major damage, that would be obvious by listening.  I’ve had “experts” examine and pronounce healthy, cartridges that sound worn or defective.  It is particularly hard to see and interpret wear on narrow contact cartridges (e.g., microline, microridge, van den hull, replicant 100, shibata) because you need high power magnification (which means poor depth of field so only a portion of the stylus is in focus), appropriate lighting, and an expert eye to interpret what is visible.

If you have doubts about a cartridge, either decide to replace it or the stylus before risking record damage or send it to the manufacturer or companies like SoundSmith or Wallytools.

You could also get a test record that has increasing modulation levels (loudness) and you could note when you hear the onset of mis-tracking and how severe is that mis-tracking.  Later, you can compare that result to what you hear when you are looking for signs of aging.  The problem with a visual inspection by even a qualified dealer is that such inspection will not reveal when the suspension is going bad.  The rubber ring around the cantilever that damps unwanted vibration can go bad even if the stylus is not played, so it is possible that the cartridge has gone bad long before wear even begins to show.

To me, listening remains the best way to determine wear.